User manual ORION TELESCOPES & BINOCULARS REV C BARLOW LENSES

[. . . ] Since magnification is determined by dividing the telescope's focal length by the eyepiece's focal length, you can see that by doubling the telescope's focal length, a 2x barlow lens doubles the magnification of the system for a given eyepiece. In this way, a 2x barlow can effectively double the number of magnifications available to you from a set of eyepieces. For example, if you have eyepieces with focal lengths of 26mm, 18mm, and 10mm, using a 2x barlow will give you the equivalent of 13mm, 9mm, and 5mm eyepieces--like getting three more eyepieces for the price of one barlow! [. . . ] So a 2x barlow placed between the telescope and the diagonal provides about a 3x magnification boost. Some barlows will not properly fit after the diagonal, so this alternate placement is desirable. (Our Shorty barlow, which has a shorter barrel, fits nicely in most diagonals. ) However, be advised that for some barlow and eyepiece combinations, focus cannot be achieved with the barlow placed before the diagonal. Power Limitations While a barlow lens will increase the magnification of a telescope, there is a limit to how much magnification is useful. It depends on the telescope's aperture, the quality of the optics, and on the outdoor "seeing" conditions, which vary from night to night. Generally, Figure A: Barlow placed after diagonal Figure B: Barlow in typical reflector Figure C: Barlow placed before diagonal (typical refractor) the maximum practical magnification a telescope can deliver is about 50x per inch of aperture (diameter of the main lens or mirror). Any higher and the images seen will just be blurry and dim. Seeing conditions (i. e. , steadiness of the atmosphere) often limit the useful magnification to much less than 50x per inch of aperture. So, for example, if you have a 3" reflector, the maximum useful magnification will be about 150x under very steady sky conditions. Say the scope has a focal length of 700mm; a 10mm eyepiece will provide 70 power. Using a 2x barlow with that eyepiece would yield 140 power, which is very close to the scope's magnification limit under excellent seeing conditions, and on most nights may simply be too much. Using a 3x barlow would not be recommended because that would yield 210 power--way too much for this scope--resulting in a degraded image. You'll have to experiment on any given night to see how much power your scope and the prevailing seeing conditions will permit. Care of Your Barlow Lens Care should be exercised when using all optical and mechanical telescope accessories. Keep your barlow in a protected environment when you're not using it. If the outside of the lens should become dirty, you may want to clean it. [. . . ] This warranty does not apply if, in Orion's judgment, the instrument has been abused, mishandled, or modified, nor does it apply to normal wear and tear. This warranty gives you specific legal rights, and you may also have other rights, which vary from state to state. For further warranty service information, contact: Customer Service Department, Orion Telescopes & Binoculars, P. [. . . ]